Coating and printing processes can involve the application of a thin film of fluid material (i.e., a coating) to a substrate, such as, for example, paper, fabric, film, foil or sheet stock. In many cases, coatings are applied to improve the surface properties of the substrate, such as the substrate's appearance, adhesion, wetability, corrosion resistance, wear resistance, and scratch resistance. In various cases, including printing processes and semiconductor fabrication, coatings may also form an essential part of the finished product.
A conventional slot coating die comprises a cavity in fluid communication with an applicator slot. Pressurized fluid is introduced into the cavity, and is then extruded out of the slot onto the desired substrate. The fluid exiting the slot generally emerges into a narrow gap between the die and the substrate, thereby forming a coating bead. If the fluid emerges at a uniform flow rate per unit slot width, the coating bead being applied to the substrate has a substantially uniform shape (e.g., thickness).
Slot coating dies are therefore generally designed to provide a high-pressure build-up along the slot (i.e., between the cavity and the outlet of the slot) to control the flow rate of a fluid flowing in and from the slot. The die may be designed such that the flow from a fluid inlet of the die to the cavity experiences a much lower pressure drop than the flow along the slot (i.e., the flow from the cavity to the outlet of the slot). Fluid flow may therefore be controlled by the pressure drop along the slot so as to provide a uniform fluid flow rate per unit slot width for fluid exiting the slot.
Coating substrates with fluids having low viscosities and/or ultra-low flow rates poses various challenges. Recent attempts to coat substrates at substantially low speeds using such fluids and conventional slot coating dies have generally resulted in coatings of erratic widths. When operating under ultra-low flow conditions, for example, conventional slot coating dies may provide coatings with a non-uniform thickness that pull or “neck” inward such that the coating is relatively thick in the center and relatively thin along the edges, thereby requiring the application of edge guides (i.e. mechanical aids to draw the dispensed material back to a substantially uniform and predictable width). In other words, without an edge guide, the width of the coating bead may become less than the width of the slot, forming a weak bead that may fail. Such difficulties make it hard to establish a bead when a coating is first applied to a substrate, resulting in slow start up times and wasted materials, including coating material and substrates.
There exists a need, therefore, for a coating technique and die that will maintain a relatively high pressure drop along the slot under ultra-low flow conditions. There also exists a need for a coating technique and die that may dispense fluids having low viscosities and/or ultra-low flow rates at a uniform flow rate, providing a fluid coating that maintains its widthwise uniformity and a coating bead that maintains its integrity without the need for edge guides. There also exists a need for a slot coating die that provides quick start up times for coating and printing processes.